astrobites: Daily Paper Summaries 2019

[Ann]

Looking Deeper at Milky Way Satellite Dwarf Galaxy Candidates
astrobites | Daily Paper Summaries | 2019 Feb 13

Dwarf galaxies are believed to be the fundamental building blocks of larger galaxies like our Milky Way and are the most abundant galaxies in our known universe. Astronomers use the terms “satellite” or “companion” galaxy to denote less massive galaxies that orbit around a larger galaxy’s center of mass. Two of the most well-known dwarf galaxies are the Large and Small Magellanic Clouds—satellite galaxies of the Milky Way. Though vitally important to galaxy formation and evolution, astronomers have a difficult time producing adequate numbers of them in cosmological simulations. Computer simulations that test the Lambda Cold Dark Matter (ΛCDM) model—the standard model of Big Bang Cosmology—predict that there should be at least 100, and possibly up to a 1000, satellite galaxies orbiting the Milky Way. However, there are only about 50 known Milky Way companions. This huge discrepancy between theory and observation has been dubbed the “missing satellites problem.” Solutions to this problem are a hot topic of debate in the astronomy community.

Meanwhile, increasingly sensitive data are revealing more Milky Way dwarf satellite candidates. Recent work done in two separate papers led by K. Bechtol and A. Drlica-Wagner revealed 16 new Milky Way dwarf satellite candidates in ~5000 square degrees of the southern sky. These candidates were found by analyzing astronomical data catalogs from the Dark Energy Survey and identifying overdensities of stellar populations. Although these 16 overdensities were previously unidentified, it is still unclear as to whether they are dwarf satellite galaxies or star clusters. By gathering more light to see even fainter stars, the subject of today’s Astrobite uses “deeper” images than the Dark Energy Survey to further investigate the exact nature of four of these stellar populations. These deeper data give more information to the authors and allow them to make better estimations as to whether they are dwarf galaxies or star clusters.

On the Nature of Ultra-faint Dwarf Galaxy Candidates.
III. Horologium I, Pictor I, Grus I, and Phoenix II ~ Helmut Jerjen et al

• arXiv.org > astro-ph > arXiv:1809.02259 > 07 Sep 2018

An irritating aspect of this astrobite is that it doesn't explain the difference between faint dwarf galaxies and stars clusters. Would the main difference be that the members of a star cluster would all be more or less the same age, whereas there should be more than one generation of a stars in a galaxy?

Ann

[bystander]

The future is bright: a new technique finds variability in K2 data of the Seven Sisters
astrobites | Daily Paper Summaries | 2019 Feb 15

In astronomy, we can’t control how stars behave, only watch from afar. So in order to get good constraints on stellar properties and build accurate stellar models, we need to study stars using multiple techniques, such as both space- and ground-based photometry, spectroscopy and interferometry.

This is certainly the case for results from NASA’s Kepler exoplanet hunting mission and its continuation K2, which – after failure of one of Kepler’s reaction wheels – observed various fields around the ecliptic. The many exoplanet detections made with Kepler and K2 need to be confirmed using follow-up observations from the ground. Similarly, Kepler’s huge wealth of data used in asteroseismic analysis – the study of stellar pulsations – requires input from ground based spectroscopic surveys. ...

This was the motivation behind halo photometry, a new technique to obtain high quality data for extremely bright stars observed with K2 around the ecliptic. An international group of astronomers, led by Tim White, developed a method that built upon previous work by Aerts et al. (2017), who found that they could recover a bright star’s signal from light scattered onto nearby pixels. They expand upon this by weighting contributions from individual pixels in a ‘halo’ around the star. By weighting the pixels based on the spacecraft systematics and signal losses present in each pixel, they can simultaneously obtain the signal from the star and perform systematics correction, which are usually done in two separate steps. Doing so is incredibly valuable for K2, where the loss of the reaction wheel caused it to perform periodic rolling motions, affecting the signal. ...

Beyond the Kepler/K2 bright limit: variability in the seven brightest members of the Pleiades ~ T. R. White et al

• Monthly Notices of the RAS 471(3):2882 (Nov 2017) DOI: 10.1093/mnras/stx1050
  arXiv.org > astro-ph > arXiv:1708.07462 > 24 Aug 2017

[bystander]

Type Ia Supernovae Could Use Some More Color
astrobites | Daily Paper Summaries | 2019 Feb 18

Type Ia supernovae (SNe Ia) are brilliant stellar explosions that are very important for cosmology. Their standardized luminosities are one of the primary tools that we can use to calculate how fast the universe is expanding! However, SNe Ia do not come standardized. There is variation among their observed properties, and these differences must be corrected through empirical relationships in order to use SNe Ia as distance indicators. In today’s paper, the authors explore whether the properties of the supernova host galaxies can yield another empirical relationship to better standardize SNe Ia. ...

Today’s paper suggests that a new correction needs to be added to our distance calculations using SNe Ia light curves. Normal corrections applied to SNe Ia are based on two factors: stretch, which is how fast the explosion declines in brightness; and color, which is essentially B-V color at peak brightness. These corrections enable us to get pretty consistent standardized luminosities for SNe Ia; the largest modern samples have scatters of only around 1% or so at peak brightness. However, the authors of today’s paper suggest that another correction for the color of the local environments of the supernovae should be included. In other words, they believe that we should not just consider the supernova itself – we should also consider its surroundings. Examples of local regions that are under consideration are shown in Figure 2. ...

Dependence of Type Ia supernova luminosities on their local environment ~ Matthieu Roman et al

• Astronomy & Astrophysics 615:A68 (Jul 2018) DOI: 10.1051/0004-6361/201731425
  arXiv.org > astro-ph > arXiv:1706.07697 > 23 Jun 2017 (v1), 05 Mar 2018 (v2)

[BDanielMayfield]

Type Ia Supernovae Could Use Some More Color
astrobites | Daily Paper Summaries | 2019 Feb 18

This one should appeal to Ann, and not simply due to the mention of color in the title.

[Ann]

Type Ia Supernovae Could Use Some More Color
astrobites | Daily Paper Summaries | 2019 Feb 18

This one should appeal to Ann, and not simply due to the mention of color in the title.

It does, Bruce! Very interesting, but utterly weird!

Type Ia supernovas tend to be brighter in passive red galaxies than in blue starforming ones. Why is that???

I can think of two reasons, dust extinction and metallicity. There is normally far less dust in "red and dead" galaxies than in blue starforming ones, and dust extinction will make the supernova look fainter. But dust extinction will not affect a supernova's intrinsic brightness, and since dust also cause reddening, it should be easy to correct for it.

What remains (as far as I can think of) is metallicity. The stars of giant elliptical galaxies are typically very metal-rich, but small elliptical galaxies are often metal-poor. But many dwarf starforming galaxies are also metal-poor. I think that the Small Magellanic Cloud is a typical example.

How utterly weird it is that red galaxies typically produce brighter supernovas than blue ones! I would love to know why that is so.

Ann

[BDanielMayfield]

How utterly weird it is that red galaxies typically produce brighter supernovas than blue ones! I would love to know why that is so.

The likely too simple to be true answer is contrast. Bright SN would stick out like a sore thumb in red galaxies, making them easy to find. SN in blue galaxies might go unnoticed, lost in the glare.

Bruce

[bystander]

The Search for Black Hole Teenagers
astrobites | Daily Paper Summaries | 2019 Feb 19

Most, if not all, galaxies contain a super-massive black hole (SMBH) at their center. But where did these giants come from? Astronomers know that less massive black holes can form when a star collapses in on itself. These stellar-mass black holes can have the mass of a few dozen suns, but they do not come close to the millions or billions of solar masses a SMBH has.

There are a few ideas for where SMBHs may have come from. One scenario is that the earliest stars left behind small baby black holes, which over time merged together to form more and more massive grown-up black holes which now reside at the center of galaxies. If this were the case, we’d expect to see some teenage, intermediate mass black holes (IMBH) — black holes larger than those born from stars but smaller than those we see in the center of galaxies.

Another possibility is that large gas clouds in the early universe collapsed to form massive black hole “seeds” the size of hundreds of thousands of stellar masses. We then wouldn’t expect to see an adolescent phase of intermediate mass black holes, as SMBHs would start growing from these large seeds instead of small ones left behind by stars.

The authors of today’s paper wanted to see if they could find IMBHs, and thus evidence for the first scenario for SMBH formation. ...

A Population of Bona Fide Intermediate Mass Black Holes Identified
as Low Luminosity Active Galactic Nuclei ~ Igor V. Chilingarian et al

• Astrophysical Journal 863(1):1 (2018 Aug 10) DOI: 10.3847/1538-4357/aad184
  arXiv.org > astro-ph > arXiv:1805.01467 > 03 May 2018

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[bystander]

A clash of disks: the traumatic life of the HV-DO Tau system
astrobites | Daily Paper Summaries | 2019 Feb 20

Astronomy 101 tells us that stars are born in molecular clouds. While this statement is certainly true, many details of how exactly this happens are still nebulous. For example, hierarchical formation theory suggests that stars prefer to be born in big families, comprising of several stars. These young multiple systems are often unstable, and over time they tend to gravitationally interact and disperse, losing all evidence of their common past.

However, if some of the family members were surrounded by protoplanetary disks at the time of this dynamical restructuring, it is possible that some evidence of their past got imprinted in the disks. Finding disks with such ’fingerprints’ could help us cast light on the initial phases of stellar formation, and could support the hierarchical formation theory.

The authors of today’s paper believe to have found a system that shows evidence of a past as part of a higher multiplicity system! ...

Evidence of a Past Disc-Disc Encounter: HV and DO Tau ~ Andrew J. Winter, Richard A. Booth, Cathie J. Clarke

• Monthly Notices of the RAS 479(4):5522 (Oct 2018) DOI: 10.1093/mnras/sty1866
  arXiv.org > astro-ph > arXiv:1807.04295 > 11 Jul 2018

[bystander]

Searching for Gravitational Waves from Binary
Supermassive Black Holes using Cosmic Lighthouses
astrobites | Daily Paper Summaries | 2019 Feb 21

When people think of gravitational wave detection, they usually think of LIGO, the giant interferometer that has successfully detected gravitational waves multiple times from merging black holes and merging neutron stars. However, there is another global collaboration that aims to detect gravitational waves using a different method. NANOGrav, or the North American Nanohertz Observatory for Gravitational Waves, is part of a worldwide effort to detect gravitational waves using the times of arrival of light from massive neutron stars that act as lighthouses in the cosmos. ...

The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries ~ K. Aggarwal et al

• arXiv.org > astro-ph > arXiv:1812.11585 > 30 Dec 2018

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[bystander]

A New Job for a Neural Net: Identifying Craters
astrobites | Daily Paper Summaries | 2019 Feb 25

Looking up at the Moon, our nearest neighbor in space, we see a desolate landscape, full of mostly just craters. Other bodies in the solar system are similar, their surfaces marked by the impact of past meteors; on places like Mercury and the Moon, there isn’t an atmosphere to drive eroding winds or other processes that could erase evidence of craters, so we see them as a well-preserved record of geologic history. These records of the past are useful to understanding the story of our solar system, informing us of when lots of craters were flying around the solar system (such as in the Late Heavy Bombardment) and even of the size of these impactors. This helps to fill in the record of how our solar system formed, and what it would have looked like at various times in its past. ...

In this paper, Lunar Crater Identification via Deep Learning, the authors apply a convolutional neural network to the problem of crater identification, specifically using data from the Lunar Reconnaissance Orbiter (LRO). Using a code they created and made available on github, they trained their CNN on human-created crater catalogs, providing the neural net with examples of the “output target” that should be created for the known data set. An example of these inputs and outputs for the LRO data is shown below in Figure 1 from the paper. ...

Lunar Crater Identification via Deep Learning ~ Ari Silburt et al

• Icarus 317:27 (Jan 2019) DOI: 10.1016/j.icarus.2018.06.022
• arXiv.org > astro-ph > arXiv:1803.02192 > 06 Mar 2018 (v1), 12 Nov 2018 (v3)

[bystander]

Solid As a Rock…Not
astrobites | Daily Paper Summaries | 2019 Feb 26

It’s not an easy life being a hot Jupiter. Besides their eternal loss of privacy from being on the hitlist of astronomers since the very moment they are detected, theirs is a tale of intense drama. The extreme radiation and tidal flexing they experience due to their proximity to their host stars make them ideal targets for studying planetary science in extreme physical conditions, particularly since there are no hot Jupiter analogues in our solar system. One of these weirdos, and also one of the most studied hot Jupiters in the last few years, is WASP-12b. In addition to being one of the hottest hot Jupiters around (with equilibrium temperature ~ 2500 K) and losing mass at an exceptionally high rate, it is also the only known hot Jupiter inspiralling rapidly towards its star. It has been speculated that sustained tidal interactions between the star and the planet could be responsible for the observed orbital decay of the planet. The authors of today’s paper investigate the case of WASP-12b in this context to understand what might be driving its orbital decay. ...

Obliquity Tides May Drive WASP-12b's Rapid Orbital Decay ~ Sarah Millholland, Gregory Laughlin

• Astrophysical Journal Letters 869(1):L15 (2018 Dec 10) DOI: 10.3847/2041-8213/aaedb1
• arXiv.org > astro-ph > arXiv:1812.01624 > 04 Dec 2018 (v1), 10 Dec 2018 (v2)

[bystander]

Finding a New Molecule: Discovery of an Organic Acid in a Protoplanetary Disk
astrobites | Daily Paper Summaries | 2019 Feb 27

Somewhat surprisingly, space is anything but empty from a chemical point of view. Astronomers have discovered more than 200 different molecules in space – around stars, in comets, throughout molecular clouds, and even in nearby galaxies. One of the most intriguing places where molecules have been detected is in the cold gas around protoplanetary disks. Protoplanetary disks, which are composed of dense gas and dust rotating around a newly-formed star, are the sites of planet formation. Understanding the chemistry occurring in these disks is especially interesting since this chemistry influences the potential for life on resulting planets. While comparatively fewer types of molecules (now about 20) are known to exist in disks, the high sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA) is making new discoveries at an increasing rate. In today’s astrobite, we take a look at the first detection of an organic acid in a protoplanetary disk and the implications for the origins of life outside of our solar system. ...

First Detection of the Simplest Organic Acid in a Protoplanetary Disk ~ Cécile Favre et al

• Astrophysical Journal Letters 862(1):L2 (2018 Jul 20) DOI: 10.3847/2041-8213/aad046
• arXiv.org > astro-ph > arXiv:1807.05768 > 16 Jul 2018

[bystander]

Searching for Earth-like worlds with a fine-toothed (Astro)comb
astrobites | Daily Paper Summaries | 2019 Feb 28

Our generation is the first to know with certainty that not only do other planetary systems exist, they are common throughout the Galaxy. The detailed characterization of these worlds offers up an extremely exciting possibility – the detection of life elsewhere in the Universe. However significant technological challenges remain for the detection of true Earth-like planets orbiting in habitable zones, the region where liquid water might exist on a planet’s surface. Now, a team of researchers from the National Institute of Standards and Technology (NIST), Pennsylvania State University and the University of Colorado has combined a near-infrared spectrograph and a new laser frequency ‘Astrocomb’ to dramatically improve the precision of infrared radial velocity measurements used to detect planets around nearby stars. ...

Stellar Spectroscopy in the Near-Infrared with a Laser Frequency Comb ~ Andrew J. Metcalf et al

• Optica 6(2):233 (20 Feb 2019) DOI: 10.1364/OPTICA.6.000233
• arXiv.org > astro-ph > arXiv:1902.00500 > 01 Feb 2019

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[bystander]

A Fresh Take on Our Moon’s Origin
astrobites | Daily Paper Summaries | 2019 Mar 01

It’s hard graduating and having your kick-ass group of STEM gal pals disperse across the country as we each pursue our STEM dreams. But it makes it a bit easier when our group chat goes off once in a while with messages like this:

• have yall seen this new theory of how the moon formed?? …honestly im shook

Our Moon is quite unique. One of the most notable properties of the Earth-Moon system is that the Moon is huge compared to the Earth. In fact, the Earth and Moon have the highest planet/moon size ratio in the solar system. The Moon is about 30% of the Earth’s radius, while Titan is about 7% of Saturn’s radius, and Ganymede is about 6% of Jupiter’s. Something special had to happen for Earth to obtain such a large satellite. For a long while now, the most popular theory has been the Giant Impact Hypothesis. It is based on the idea that while the solar system and planets were forming, a body about the mass of Mars (hypothetically named Theia) collided with a proto-Earth. After the collision, the remains of Theia became the Moon. This collision theory explains why the Moon is much bigger than Earth (as well as a few other things). However, one of its weakest points is the fact that the Earth and Moon have nearly identical chemical make-ups. If the Theia was formed elsewhere in the solar system, it – and thus the Moon – would contain very different elements and molecules than the Earth.

Today’s paper offers an alternative view of how the Moon came to be, and the authors are able to explain away the Giant Impact’s weakest parts. Their theory also starts with a collision, although the collider does not necessarily need to be very massive, like Theia. The collision does need to have a high energy and high angular momentum (basically the collider needs to be fast and possibly spinning/rotating at a high rate). In this paper, and supporting work, the models show that this type of impact would occur more often than a Giant Impact collision. They also show that with a high energy and high angular momentum collision with a proto-Earth, a structure called a synestia can form. ...

The Origin of the Moon within a Terrestrial Synestia ~ Simon J. Lock et al

• Journal of Geophysical Research 123(4):910 (Apr 2018) DOI: 10.1002/2017JE005333
• arXiv.org > astro-ph > arXiv:1802.10223 > 28 Feb 2018

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[bystander]

The Circle of Life… of Black Holes
astrobites | Daily Paper Summaries | 2019 Mar 04

Today’s post is about a new release of a simulation called SIMBA. This is the next generation of the MUFASA cosmological simulations, with some key updates and improvements (more information on cosmological simulations here). For those keen to know the details, SIMBA uses a (100h-1Mpc)3 box and 1024³ gas elements. It uses a Meshless Finite Mass (MFM) solver and evolves dark matter and gas elements together including gravity and pressure forces.

It would be impossible to explain the vast amount of physics that goes into galaxy formation simulations in just one article, so let’s focus on what’s new and novel about the SIMBA simulations – the treatment of black holes. ...

Simba: Cosmological Simulations with Black Hole Growth and Feedback ~ Romeel Davé et al

• arXiv.org > astro-ph > arXiv:1901.10203 > 29 Jan 2019

[bystander]

A Shocking Model of FRBs
astrobites | Daily Paper Summaries | 2019 Mar 05

Ever since Fast Radio Bursts (FRBs) were first reported in the literature in 2007, they’ve remained an exciting and mysterious group of astrophysical phenomena. Until 2014, there were only six signals recorded in the literature, all from the Parkes Observatory in Australia. However, thanks to advances in technology and instrumentation, we have seen a flurry of FRB detections from various other radio observatories in the past five years, bringing the known total up to at least 65. These FRB signals have mostly been wildly energetic one-time bursts, releasing more energy in fractions of a second than our Sun does in decades.

Repeating FRB signals also exist. FRB 121102 was the first detected repeating FRB, observed to be a repeater by the Arecibo Observatory in 2015 when they detected ten FRBs at a location in the sky near a FRB they discovered three years earlier. The second repeating FRB was only recently discovered in 2018 by the CHIME/FRB collaboration during a testing phase. These repeating FRB signals provide a tantalizing window through which scientists can debate the nature and origins of FRBs. Multiple possible origins of FRBs have already been covered here on astrobites: neutron star mergers, flaring stars, and dark matter-induced collapse of neutron stars. Today’s paper focuses on the properties of FRB 121102 to suggest that FRBs may occur as laser-like emission from decelerating shock waves, preferably—but not necessarily—from magnetars, which are neutron stars with tremendously powerful magnetic fields. ...

Fast Radio Bursts as Synchrotron Maser Emission from Decelerating
Relativistic Blast Waves ~ Brian D. Metzger, Ben Margalit, Lorenzo Sironi

• arXiv.org > astro-ph > arXiv:1902.01866 > 05 Feb 2019

[bystander]

Stars that exploded just a little bit
astrobites | Daily Paper Summaries | 2019 Mar 06

In today’s bite, we are going to talk about some pretty weird stars.

A couple of years ago, a white dwarf named LP40-365 was identified as a high-velocity star with a very unusual chemical composition. It was proposed that the star is a partially burnt white dwarf that had survived a (subluminous) type Ia supernova. Basically, the white dwarf went supernova but was not completely destroyed in the explosion, resulting in a fainter supernova that left a remnant behind.

This, put mildly, is super exciting stuff! We still don’t know whether type Ia supernovae result from the thermonuclear explosion of a white dwarf accreting material from a main-sequence companion, or from the merger of two white dwarfs. Observing a surviving remnant can reveal a lot about how thermonuclear supernovae actually work.

Today’s paper reports the discovery of three more stars that are similar to LP40-365, all supernova survivors. The authors selected high-velocity stars by mining Gaia DR2 and took their spectra. By analyzing stellar spectra, one can identify various elements present in the star’s atmosphere. ...

A class of partly burnt runaway stellar remnants from peculiar thermonuclear supernovae ~ R. Raddi et al

• arXiv.org > astro-ph > arXiv:1902.05061 > 13 Feb 2019

[bystander]

A New Window into Prebiotic Nitrogen Chemistry in Protoplanetary Disks
astrobites | Daily Paper Summaries | 2019 Mar 07

Did you know that you are made out of primordial dust and gas? It’s true! Our solar system formed from a gravitationally-collapsing cloud of interstellar dust and hydrogen gas, forming a proto-Sun in the center of this hot dense material. The various planets are then thought to have formed out of the material in the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun’s formation, which ultimately takes the shape of a rotating protoplanetary disk. As discussed in a previous astrobite, these disks exhibit an intriguing variety of chemistry that is only beginning to be probed with the higher sensitivity of telescopes such as ALMA. Understanding how the early inventory of organic molecules that are present in this stage of planet formation developed into the vast complexity of biochemistry we see today is key to the study of the origins of life.

Chemists and astronomers alike have been especially interested in nitrile-bearing molecules, which contain a carbon-nitrogen triple bond. These molecules likely play a crucial role in prebiotic chemistry, as recent studies have shown that this particular bond is involved in prebiotic synthesis of RNA and protein precursors. In today’s astrobite, we take a look at these important types of molecules in a recent survey of protoplanetary disks and the implications for nitrogen-based chemistry in our early solar system. ...

A Survey of CH₃CN and HC₃N in Protoplanetary Disks ~ Jennifer B. Bergner et al

• Astrophysical Journal 857(1):69 (2018 Apr 10) DOI: 10.3847/1538-4357/aab664
• arXiv.org > astro-ph > arXiv:1803.04986 > 13 Mar 2018

[bystander]

Hunting for Stellar Streams
astrobites | Daily Paper Summaries | 2019 Mar 11

“The discovery of a new dish confers more happiness on humanity, than the discovery of a new star” goes a saying by the famous French food philosopher Jean Anthelme Brillat-Savarin. I confess that, even as an astronomer, I find it hard to refute this claim. But some astronomical discoveries provide so much food (for thought), that I wonder if Brillat-Savarin could have been convinced to broaden his definition of a dish. Today’s paper, for example, demonstrates that there are few things in the world that make a person who studies galactic dynamics as happy as the discovery of a new stellar stream.

Stellar streams are the by-products of violent interactions between a galaxy and a smaller passer-by, typically a dwarf galaxy or a globular cluster. And though stellar streams have been observed around many other galaxies, we need look no further than our own Milky Way to see some truly majestic stream-like structures (as shown Figure 1). ...

Identification of the Long Stellar Stream of the Prototypical Massive Globular Cluster ω Centauri ~ Rodrigo Ibata et al

• arXiv.org > astro-ph > arXiv:1902.09544 > 25 Feb 2019

[bystander]

Look! A Lensing!
astrobites | Daily Paper Summaries | 2019 Mar 12

Gravitational lensing is a peculiar thing. It involves a massive object, called a lens, standing between a source and an observer. The lens bends light from a source for an observer to see as a ring or bright points arranged around the lens (see Figure 1). The degree and type of distortion is governed by the mass of the lens as well as the distances between the source, the lens, and the observer.

Gravitational lensing can occur on a variety of scales. Strong lensing produces the dramatic rings and multiple images, employing massive lenses (such as galaxies or galaxy clusters) to do so. Weak lensing is more subtle, also having massive lenses distort the images of source galaxies very slightly. This slight distortion makes it seem as though the source galaxies are aligned with some field—like iron filings in the presence of a magnet. Microlensing involves much smaller lenses, such as stars with planets, and any distortions cannot be directly observed. Instead, microlensing results in the source appearing to brighten and dim with a sharp tell-tale peak in its light curve.

All forms of lensing are useful. Microlensing in particular can be used to detect exoplanets and measure the masses of the lens stars very precisely (see this Astrobite for an example of that). In this paper, the authors take advantage of the second data release from the Gaia satellite to predict instances of astrometric microlensing, or when the distortion from a lens causes the source to appear to shift position. ...

Prediction of Astrometric Microlensing Events from Gaia DR2 Proper Motions ~ J. Klüter et al

• Astronomy & Astrophysics 620:A175 (Dec 2018) DOI: 10.1051/0004-6361/201833978
• arXiv.org > astro-ph > arXiv:1807.11077 > 29 Jul 2018 (v1), 16 Oct 2018 (v2)

[bystander]

Putting Supermassive Black Holes on the Scale
astrobites | Daily Paper Summaries | 2019 Mar 13

One of the most infamous solutions to Einstein’s General Relativity, black holes have been realized in the modern era as a fundamental aspect of our Universe. It is now widely believed that a supermassive black hole (SMBH) resides in the center of nearly every galaxy. What’s more is that over the past two decades, evidence has been building which suggests the lives and deaths of galaxies are dictated in part by their SMBH. This may seem obvious, but SMBHs are incredibly tiny compared to the vastness of a galaxy. To put this into perspective in terms of relative size, it is as if a single atom interferes with your balance as you take a morning jog!

Early observations of galaxies revealed a population with intensely bright central regions called Active Galactic Nuclei (AGN). When measured with spectroscopy, the emission line features from the AGN are bizarre. A set of extremely broad lines are thought to be Doppler broadened due to fast-moving gas clouds near the SMBH, and a set of narrow lines from slower moving photo-ionized clouds further out. Hence, these regions are named the Broad Line Region (BLR) and Narrow Line Region (NLR), respectively. ...

The authors of today’s astrobite demonstrate a powerful new way to estimate the masses of SMBHs in Type II AGN. By exploiting a previously unknown relationship between the BLR and the NLR emission lines, they obtain SMBH masses for systems which before now only had measurements of galaxy bulge properties. ...

Black Hole Mass Estimation for Active Galactic Nuclei from a New Angle ~ Dalya Baron, Brice Ménard

• arXiv.org > astro-ph > arXiv:1903.01996 > 05 Mar 2019

[bystander]

Halo assembly bias rides a gravitational tide
astrobites | Daily Paper Summaries | 2019 Mar 14

Dark matter halos make up the underlying structure of the universe. These halos, spherical-ish clouds of gravitationally bound dark matter, are connected by dark matter filaments. Together these make up a giant cosmic web.

It turns out that halos harbor a bias ... they care about where they are situated in this greater environment. Specifically, the mass of the halo is related to the density (or “clustering”) of the cosmic web on much larger scales than the size of halo itself. This is because halos are formed by matter from these background densities collapsing, so larger halos will form in more dense environments.

Even at a fixed halo mass, the clustering depends on other “internal” halo properties. One of these is the concentration of the halo, which is illustrated in Figure 1 on a slice of a dark matter simulation. You can see visually that halos with higher concentration (red dots, left panel) cluster less strongly than halos with lower concentration (green dots, right panel). This effect is known as assembly bias.

Assembly bias also affects halo properties including their shape, spin, and the velocity distribution. Today’s paper introduces a new way of connecting all of these properties to the large-scale environment, via gravitational tides. ...

Cosmic web anisotropy is the primary indicator of halo assembly bias ~ Sujatha Ramakrishnan et al

• arXiv.org > astro-ph > arXiv:1903.02007 > 05 Mar 2019

[bystander]

New Horizons Images Show Fewer Small Kuiper Belt Objects than Expected
astrobites | Daily Paper Summaries | 2019 Mar 15

The population of small objects in the Kuiper Belt is of interest because these are the last surviving remnants of the disk from which the Solar System formed. Clues to the processes that govern how planets grow and evolve are hidden among the objects beyond Neptune, yet the composition of the Kuiper Belt is largely unknown. Specifically, astronomers want to determine the size-frequency distribution of Kuiper Belt Objects (KBOs)—how many objects of each size there are—to infer the dominant planetary system dynamics over billions of years.

The larger KBOs, including Pluto, can be observed from ground-based telescopes, but KBOs smaller than a few kilometers in diameter are nearly impossible to detect. Without being able to sample the Kuiper Belt, it is a challenge for astronomers to determine the distribution of KBOs. New Horizons provided a unique opportunity to study the Kuiper Belt when it successfully flew past Pluto and Charon in 2015, returning the first high-resolution images ever.

Astronomers around the world were excited by the results of the flyby and news media spread imagery of Pluto’s icy heart, but geologists took the opportunity to study the geology of Pluto and Charon for the first time. The authors of this paper have yet another goal: to use impact craters on Pluto and Charon to determine the distribution of KBOs. ...

Impact Craters on Pluto and Charon Indicate a Deficit of Small Kuiper Belt Objects ~ K. N. Singer et al

• Science 363(6430):955 (01 Mar 2019) DOI: 10.1126/science.aap8628
• arXiv.org > astro-ph > arXiv:1902.10795 > 27 Feb 2019

[bystander]

How irregular rotation can change a galaxy’s metal content
astrobites | Daily Paper Summaries | 2019 Mar 18

Space is a battleground for giants. When galaxies drift too close to one another, their immense gravitational influences can bend, deform, or even tear apart the structure of each other. In the most extreme cases, the galaxies will collide and merge into single structures. Unfortunately, mergers happen over hundreds of millions of years and cannot be observed in their entirety. Luckily, the Universe provides so many galaxies that we can combine snapshots (Figure 1) of many different mergers with simulations to understand the process. Extracting the history of individual galaxies is much more difficult. Mergers between two large galaxies of similar size (major mergers) can result in distinct elliptical galaxies, but a small galaxy being subsumed into a much larger one (a minor merger) leaves a significantly smaller mark. Today’s paper studies simulations of collided galaxies to try and find observational signatures of galaxies that are still reeling from minor mergers in the past.

The predecessor of today’s paper is a different paper by the same authors from a year ago. There, they discuss results from a cosmology simulation that allows for the formation and subsequent collisions of galaxies. Dark matter filaments, thin strands of dark matter that connect distant galaxies, are also included in the simulation. These filaments are of particular importance because as a galaxy passes through a dark matter filament, gas along the filament will accrete into the galaxy. ...

The metallicity and elemental abundance maps of kinematically atypical galaxies
for constraining minor merger and accretion histories ~ Philip Taylor, Chiaki Kobayashi, Christoph Federrath

• Monthly Notices of the RAS 485(3):3215 (May 2019) DOI: 10.1093/mnras/stz630
• arXiv.org > astro-ph > arXiv:1903.01597 > 05 Mar 2019

[bystander]

Darwin’s Theory of Evolution, but make it planets
astrobites | Daily Paper Summaries | 2019 Mar 19

It was just a few decades ago that humans had knowledge only of planets within our own solar system. Now, exoplanets are known to be quite widespread. Old missions like Kepler and current missions like TESS continue to contribute to our understanding of the qualities of exoplanets, but other questions remain. What sort of environments are conducive to planet formation and retention? In what locales may a sustained planetary system be unlikely or even impossible? Today’s paper considers an extreme location for a planetary system: Star clusters.

Among all 4000+ exoplanets discovered to date, fewer than 1% are found within star clusters. Only 30 exoplanets have been found within star clusters and a measly single planet has been detected within a dense globular cluster (GC). Why is this? Is there something about star clusters, and GCs in particular, that makes their environment uniquely inhospitable for planets to form? Or are our telescopes just missing them somehow? Today’s paper uses numerical simulations to study if the unique conditions within star groups that give rise to GCs, called young massive star clusters (YMCs), may influence the types of planets that can be retained within these gravitationally complex systems. ...

On the Survivability of Planets in Young Massive Clusters ~ Maxwell Xu Cai et al

• arXiv.org > astro-ph > arXiv:1903.02316 > 06 Mar 2019